Birkeland current

Auroral Birkeland currents carry about 100,000 amperes during quiet times and more than 1 million amperes during geomagnetically disturbed times. Birkeland had estimated currents "at heights of several hundred kilometres, and strengths of up to a million amperes" in 1908. (similar to the Kelvin–Helmholtz instability), that subsequently leads to filamentation. Such vortices can be seen in aurora as "auroral curls". Birkeland currents are also one of a class of plasma phenomena called a z-pinch, so named because the azimuthal magnetic fields produced by the current pinches the current into a filamentary cable. This can also twist, producing a helical pinch that spirals like a twisted or braided rope, and this most closely corresponds to a Birkeland current. Pairs of parallel Birkeland currents will also interact due to Ampère's force law: parallel Birkeland currents moving in the same direction will attract each other with an electromagnetic force inversely proportional to their distance apart whilst parallel Birkeland currents moving in opposite directions will repel each other. There is also a short-range circular component to the force between two Birkeland currents that is opposite to the longer-range parallel forces. Electrons moving along a Birkeland current may be accelerated by a plasma double layer. If the resulting electrons approach the speed of light, they may subsequently produce a Bennett pinch, which in a magnetic field causes the electrons to spiral and emit synchrotron radiation that may include radio, visible light, x-rays, and gamma rays. == Spatial distribution and responses to solar wind disturbances ==

Auroral Birkeland currents are constrained along the geomagnetic field. Therefore, the current’s distribution in 3-dimensional space could be largely described using the 2-dimensional distribution of the current’s footprints at a given altitude in the ionosphere, e.g., 110 km. A classical 2-dimensional description was summarized from satellite observations by Iijima and Potemra. The footprints of Auroral Birkeland currents exhibit ring-shaped structures. As the currents are driven by solar winds, their spatial distribution and intensity are also dynamically moderated by solar wind disturbances. Under intensive solar wind disturbances, the rings can quickly shift by 10 degrees in latitude in about 10 minutes. The latitudinal shift takes on average 20 minutes to respond to a solar wind change during the daytime but 70–90 minutes at night. ==History==

. In 1939, the Swedish Engineer and plasma physicist Hannes Alfvén promoted Birkeland's ideas in a paper published on the generation of the current from the Solar Wind. In 1964 one of Alfvén's colleagues, Rolf Boström, also used field-aligned currents in a new model of auroral electrojets. Proof of Birkeland's theory of the aurora only came after a probe was sent into space. The crucial results were obtained from U.S. Navy satellite 1963-38C, launched in 1963 and carrying a magnetometer above the ionosphere. In 1966 Alfred Zmuda, J.H. Martin, and F.T.Heuring analysed the satellite magnetometer results and reported their findings of magnetic disturbance in the aurora. In 1967 Alex Dessler and graduate student David Cummings wrote an article arguing that Zmuda et al. had detected field-aligned currents. Alfvén subsequently acknowledged that Dessler had "discovered the currents that Birkeland had predicted" and they should be called Birkeland-Dessler currents. 1967 is therefore taken as the date when Birkeland's theory was finally acknowledged to have been vindicated. In 1969 Milo Schield, Alex Dessler and John Freeman used the name "Birkeland currents" for the first time. In 1970 Zmuda, Armstrong and Heuring wrote another paper agreeing that their observations were compatible with field-aligned currents as suggested by Cummings and Dessler and by Boström. ==See also==